Two-dimensional (2D) zeolite nanosheets find uses in catalysis and separations as building blocks for the fabrication of membranes, catalysts, nanodevices, films etc. Currently, conventional preparation of 2D zeolite nanosheets relies on: 1) the availability of layered zeolite precursors; and 2) development of efficient exfoliation methods by which the layered zeolite precursors can be delaminated and dispersed in a solvent.
Zeolite Socony Mobil-5 (framework type MFI from ZSM-Five) is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Recently, multi-lamellar MFI (ML-MFI) zeolites, which feature 1.5-unit-cell MFI nanosheets stacked along the b-axis have been synthesized in the presence of a class of organic structural directing agents (OSDA) composed of long-chain alkyl groups and two quaternary ammonium head groups spaced by a C6 alkyl linkage (C22-6-6). The bi-ammonium head group possibly plays a structure-directing role for the formation of the MFI crystal structure, while the long-chain alkyl tails possibly act as a surfactant favoring the formation of lamellar assemblies. ML-MFI can be exfoliated to obtain 2-dimensional zeolite MFI nanosheets. However, the strong van der Waals force among long-chain alkyl groups (C22) between the MFI layers hinders the exfoliation, greatly different from the case of typical layered materials such as clays, graphene, MoS2, BN, and WS2 etc., in which the layers are electrostatically bonded by weak van der Waals interactions or hydrogen bonds, allowing exfoliation methods such as solvation of interlayered cations, ultrasonic methods, stirring, and freeze-thawing method etc.
A polymer-melt-compounding technique to exfoliate layered zeolite precursors has also been reported. To achieve exfoliation, polystyrene (PS) was mixed with ML-MFI (or other layered zeolite) and melt-blended. Suspensions of exfoliated 2D MFI nanosheets were obtained after removing PS by washing the composite with toluene followed by redispersion in octanol. It is believed that this is currently the only method that can provide high purity 2D MFI zeolite nanosheets suspensions. However, the polymer-melt-compounding technique tends to damage and fragment nanosheets into smaller pieces (˜200 nm basal dimensions) and is a multi-step time-consuming and low-yield process. Moreover, the obtained MFI nanosheets are hydrophobic and can be dispersed in non-polar solvents like toluene and octanol but not in water, which is desirable because it may enable layer-by-layer deposition methods for thin film and nanocomposite formation.
Treatment with acid has been demonstrated to remove some of the SDA and may allow dispersion of the nanosheets in ethanol. However, the acid extraction process only partially removed the OSDA molecules. Even for exfoliated MFI nanosheets, OSDAs were still present and the partially detemplated nanosheets could not be dispersed in water. Moreover, the pores of the MFI framework remain occupied by OSDA (as indicated by the absence of microporosity probed by standard porosimetry methods like cryogenic Ar-adsorption) prohibiting their use as a molecular sieve without further treatment for OSDA removal.